Make application work without tmp directory

The tmp directory is added to version control in the newly created
application. This was added in Rails 5.0.0(f06ce4c12a).

However, applications created before that are not guaranteed to have the
tmp directory. If the tmp directory does not exist, writing to the key file
raise error.

This is a bit incompatible. So I fixed that create the directory before
writing a key.

railties/lib/rails/application.rb

@@ -587,6 +587,7 @@ def generate_development_secret
 
           if !File.exist?(key_file)
             random_key = SecureRandom.hex(64)
+            FileUtils.mkdir_p(key_file.dirname)
             File.binwrite(key_file, random_key)
           end
 

railties/test/application/configuration_test.rb

@@ -601,6 +601,9 @@ def index
         Rails.application.credentials.secret_key_base = nil
       RUBY
 
+      # For test that works even if tmp dir does not exist.
+      Dir.chdir(app_path) { FileUtils.remove_dir("tmp") }
+
       app "development"
 
       assert_not_nil app.secrets.secret_key_base